β2-GPI is a major antigen which is present in patients with antiphospholipid syndrome (APS) and is recognized by “an antiphospholipid antibodies.” β2-GPI is known to specifically bind to oxidized low-density lipoprotein (oxidized LDL; oxLDL), but not to non-oxidized (native) low-density lipoprotein (LDL). PCT International Patent Publication (pamphlet) WO 95/9363 discloses an oxLDL assay method based on a specific binding property with respect to β2-GPI, a kit for diagnosing an arteriosclerotic disease employing the assay method, etc.
However, the portion of the oxLDL structure which β2-GPI recognizes for binding has not been identified.
Therefore, identification of the portion of the oxLDL structure to which β2-GPI specifically binds would realize an assay of β2-GPI or a similar assay by use of an easier and simpler system employing a substance including the portion. In addition, handling and storage of the reagents used in the assay would be further facilitated, leading to provision of constant-quality assay reagents, assay kits, etc. at low cost.
In view of the foregoing, the present inventors have carried out extensive studies in order to provide a substance having a structure which specifically binds to β2-GPI; a very practical assay method for β2-GPI or the like making use of the substance; and a very practical method for detecting a disease on the basis of the assay method. The inventors have isolated a substance having a structure which specifically binds to β2-GPI, and based on the isolated substance, have established an assay method for β2-GPI or the like as well as a method for detecting a disease employing the assay method. The present invention has been accomplished on the basis of these findings.